Optical systems by which a front object and an approximately lateral object can be simultaneously observed have been known, up to now. In this case, the range which the above term, “approximately lateral”, defines includes not only the lateral side of an optical system itself but also the diagonally forward and diagonally backward sides of the optical system.
In such optical systems, an optical system which is formed in such a way that, after light from the approximately-lateral-object side is reflected two times on the inside, the light emerges to the image side has been known (for example, refer to International Publication No. 2003/042743).